Jarvik7 Wrote:Another explanation is "new information vs old information"Whether it's new or old is irrelevant. For example,
You: I've been meaning to say this, but I-I prefer boys...
Your girlfriend: WTF.
You: I said I'm gay! I love cute boys!!
Your girlfriend: Y-You love cute boys?! Hooray! I'm a huge fan of yaoi light novels!
The "Y-You love cute boys?!" would be "可愛い男の子が好きなの?". Of course, 可愛い男の子 is old information, but still your girlfriend uses が because she thinks it's very important. What actually makes her use が is the "surprise," which makes her think 可愛い男の子 is more important. You can also see that the old-or-new rule doesn't work in the 本は好き? or 本が好き? examples in my previous post. You can't conclude if you should use は or が if you're thinking whether 本 is new or old. Here's another example:
You: I've been meaning to say this, but I'm in love with your mother...
Your girlfriend: WTF.
You: I said I love your mom!! Yes! I've been cheating on you!
Your girlfriend: Y-You love my mom?! What a coincidence! I just slept with your dad!
The "Y-You love my mom?!" would be "私のお母さんが好きなの?" because 私のお母さん is much more important than 好き in this sentence in spite of the fact that it's old information. Of course, you can replace her mom with another thing such as a proper noun.
New information is most likely important than other things, but old information can be very important, too.
As for 私はしゅんです, imagine there are two Shuns in a room. If you want to say, "I'm Shun. And he's also Shun. lol confusing!" then it'd be 私はしゅんです。そして彼もしゅんです。はは、ややこしいですね。
If you said, "私がしゅんです," then people would say "So, the other Shun is bogus" because you'd sound like you were saying "I am Shun."
Edited: 2009-06-09, 11:13 pm

I know Japanese grammar on a way more technical level than I do English. Being a native can be a curse